Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking
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METALLIC materials used in the construction of engineered components are subject to degraded performance and potential failure due to environment assisted cracking (EAC). The high costs associated with component inspection, EAC mitigation, and remediation provide economic incentive for developing quantitative EAC analysis methods for use in component design and evaluation of the reliability and remaining lifetimes of existing components. Historically, development of these methods is based on the assumptions that, for a given material-environment combination, there exists a unique threshold stress intensity factor, Kth, below which crack growth will not occur, and a unique kinetic _ _ and the relationship, aðKÞ, between crack velocity, a, applied stress intensity factor, K. The data used to _ typically have been obtained establish Kth and aðKÞ for relatively small K variations due to small crack extensions in tests run under constant load, constant M.M. HALL, Jr., formerly Consultant, Bechtel Bettis, Inc., Monroeville, PA 15146, is retired. Contact e-mail: hallmm63@ comcast.net Manuscript submitted December 3, 2009. Article published online May 29, 2010 304—VOLUME 42A, FEBRUARY 2011
displacement, and, less typically, constant applied stress intensity factor. However, engineering applications of crack growth predictive methods almost always involve assessments of potentially large crack extensions where K may undergo significant change, either increasing or decreasing, due to (1) time-dependent changes in applied forces, dK/dt; and (2) crack growth through large primary stress gradients and regions of secondary residual stress, dK/da. Crack growth analyses in both cases typically have _ been conducted using K-rate independent aðKÞ expressions to calculate and sum small increments of crack growth, incrementally adjusting K to match the variableK predicted for the crack path of interest. _ may not always be The likelihood that Kth and aðKÞ unique, except under carefully established steady-state conditions, has been recognized for over 35 years.[1–5] Hudak and Wei[2] conducted crack growth rate experiments on 4340 steel in distilled water to better understand the earlier static load results of Landis and Wei,[1] which showed that Kth is not unique and that stage I crack velocity is not a unique function of K when K increases with crack growth. Both were found to be dependent on the initial value of K. By testing specimens for which K remains constant during crack growth under constant load, Hudak and Wei concluded that the behavior observed by Landis and Wei is ‘‘due, at least in METALLURGICAL AND MATERIALS TRANSACTIONS A
part, to non-steady state crack growth.’’ The crack velocity accelerates from an initially low crack growth rate and approaches steady state after a transient period of crack growth that is dependent on the initial applied K and environment chemistry. Toribio and Kharin[6] collected from the literature numerous experimental _ are not unique but depend examples where Kth and aðKÞ on geometry, prelo
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